1. Field of Invention
This invention relates to scanned-probe microscopy, specifically to a means of locally exciting or detecting sub-visible electromagnetic fields while measuring or altering sample topography simultaneously or alternatively, using a combined scanning probe microscope (SPM) tip and electromagnetic waveguide structure integrated with the cantilever of the SPM.
2. Description of Prior Art
Near-field scanning optical microscopy (NSOM) is a technique by which visible or near-visible radiation (free-space wavelength generally less than one micrometer) is confined within a sub-wavelength-diameter aperture for localized excitation of a sample or for localized detection of electromagnetic phenomena in this wavelength regime. The principle of NSOM, namely obtaining sub-wavelength resolution in excitation or detection, is quite general, and need not be restricted to visible radiation, as was already demonstrated in Ash and Nichols, "Super-resolution aperture scanning microscope," Nature, 237 510-12 (1972) using 3-cm wavelength microwave radiation confined to a sub-wavelength aperture to achieve 1/60 wavelength-relative resolution in scanning across a sample.
Scanning probe microscopy (SPM), on the other hand, is a well-known means of characterizing the topography of a sample by scanning a sharp tip mounted on a flexible cantilever over the sample while recording a quantity such as cantilever deflection in response to local forces or a current during tunneling between tip and sample. The feedback mechanism common to all SPM implementations offers a very important benefit in maintaining a precise tip-sample distance control operating in a variety of modes, known in force microscopy as "contact", "non-contact", "periodic-contact", and "near-contact" modes. Of particular interest to the present invention is the variety of non-contact force microscopy modes in which the tip/cantilever assembly is vibrated at its resonant frequency and the change in this frequency due to surface force (e.g. electrostatic, van der Waals, or magnetic) gradients of the sample is detected and used as a feedback mechanism, allowing the tip to scan above the sample at a distance of 5 to 500 Angstroms. Several detection mechanisms with sub-Angstrom sensitivity for cantilever deflection exist, such as optical beam deflection, interferometry, piezoresistivity, changes in capacitance between the cantilever and a fixed plate, and vacuum tunneling, all described in U.S. Pat. No. 5,354,985 to Quate and in U.S. Pat. No. 5,489,774 to Akamine et al., as well as in Blanc, Brugger et al., "Scanning force microscopy in the dynamic mode using microfabricated capacitive sensors," Journal of Vacuum Science & Technology B (Microelectronics and Nanometer Structures), 14 901-5 (1996), all incorporated herein by reference. The feedback technique combined with the mechanical resonance limitations of the SFM cantilever usually limits the frequency response of the system to below 100 kHz. While there are few dynamic phenomena of interest at frequencies below 100 kHz, significant prior art exists for combining the benefits of the SFM and the visible-light NSOM, such as that in U.S. Pat. No. 5,354,985 to Quate and in U.S. Pat. No. 5,489,774 to Akamine et al., but again these inventions locally detect or excite only visible or near-visible light.
There is, however, a large class of electromagnetic phenomena having characteristic frequencies lying between the extremes of frequency described above, such as integrated-circuit (IC) electromagnetic fields (generally between 1 MHz and 100 GHz, herein termed "microwaves"), biological-membrane contrast mechanisms (primarily due to absorption of radiation by water in the range of 100 GHz to 30 THz or 10 micrometer wavelengths), molecular rotational absorptions (also primarily in the 100 GHz to 30 THz range, herein termed "FIR" for "far infrared"), and responses of material properties in the range of 1 MHz to 30 THz.
There has been prior work in the field of sub-visible microscopy. U.S. Pat. No. 4,994,818 to Keilmann and Fee, Chu et al., "Scanning electromagnetic transmission line microscope with sub-wavelength resolution," Optics Communications, 69 219-24 (1989) and Keilmann, "FIR microscopy," Infrared Physics & Technology, 36 217-24 (1995), all incorporated herein by reference, teach a scanning coaxial tip for focusing radiation. In this work, the advantages of a coaxial tip for shielding and for localizing the tip/sample interaction or stimulus/response are set forth, and the extreme sub-wavelength resolution of the coaxial tip is anticipated. There is, however, no integration of an SPM (and its associated probe positioning and topography-measuring advantages) with the coaxial tip being taught.
None of the prior art in NSOM as taught by Durig, Pohl et al., "Near-field optical scanning microscopy with tunnel-distance regulation," IBM J. Res. Develop., 30 478-83 (1986); Toledo-Crow, Yang et al., "Near-field differential scanning optical microscope with atomic force regulation," Applied Physics Letters, 60 2957-9 (1992); Lieberman, Lewis et al., "Multifunctional, micropipette based force cantilevers for scanned probe microscopy," Applied Physics Letters, 65 648-50 (1994); Moers, Tack et al., "Photon scanning tunneling microscope in combination with a force microscope," Journal of Applied Physics, 75 1254-7 (1994); Radmacher, Hillner et al., "Scanning nearfield optical microscope using microfabricated probes," Review of Scientific Instruments, 65 2737-8 (1994); Danzebrink, Wilkening et al., "Near-field optoelectronic detector probes based on standard scanning force cantilevers," Applied Physics Letters, 67 1981-3 (1995); Davis, Williams et al., "Micromachined submicrometer photodiode for scanning probe microscopy," Applied Physics Letters, 66 2309-11 (1995); Lewis, Ben-Ami et al., "NSOM the fourth dimension: integrating nanometric spatial and femtosecond time resolution," Scanning, 17 3-13 (1995); McCutchen, "Transmission line probes for scanning photon-tunneling microscopy," Scanning, 17 15-17 (1995); Mihalcea, Scholz et al., "Multipurpose sensor tips for scanning near-field microscopy," Applied Physics Letters, 68 3531-3 (1996); Ruiter, Moers et al., "Microfabrication of near-field optical probes," Journal of Vacuum Science & Technology B (Microelectronics and Nanometer Structures), 14 597-601 (1996); Noell, Abraham et al., "Micromachined aperture probe tip for multifunctional scanning probe microscopy," Applied Physics Letters, 70 1236-38 (1997), deals with sub-visible radiation, although in many of these works, the advantages of simultaneous tip-sample distance regulation, topography sensing and scanning optical microscopy were recognized.
Much prior art exists describing larger-scale field probes used for sensing radiation from an object and for measuring local material properties, as exemplified in Misra, Chabbra et al., "Noninvasive electrical characterization of materials at microwave frequencies using an openended coaxial line: test of an improved calibration technique," IEEE Transactions on Microwave Theory and Techniques, 38 8-14 (1990); Chevalier, Chatard-Moulin et al., "High temperature complex permittivity measurements of composite materials using an open-ended waveguide," Journal of Electromagnetic Waves and Applications, 6 1259-75 (1992); Osofsky and Schwarz, "Design and performance of a noncontacting probe for measurements on high-frequency planar circuits," IEEE Transactions on Microwave Theory and Techniques, 40 1701-8 (1992); Xu, Ghannouchi et al., "Theoretical and experimental study of measurement of microwave permittivity using open ended elliptical coaxial probes," IEEE Transactions on Microwave Theory and Techniques, 40 143-50 (1992); Jiang, Wong et al., "Open-ended coaxial-line technique for the measurement of the microwave dielectric constant for low-loss solids and liquids," Review of Scientific Instruments, 64 1614-21 (1993); Jiang, Wong et al., "Measurement of the microwave dielectric constant for low-loss samples with finite thickness using open-ended coaxial-line probes," Review of Scientific Instruments, 64 1622-6 (1993); Keilmann, van der Weide et al., "Extreme sub-wavelength resolution with a scanning radio-frequency transmission microscope," Optics Communications, 129 15-18 (1996); Vlahacos, Black et al., "Near-field scanning microwave microscope with 100 .mu.m resolution," Applied Physics Letters, 69 3272-4 (1996); Wei, Xiang et al., "Scanning tip microwave near-field microscope," Applied Physics Letters, 68 3506-8 (1996), so the use of such waveguide probes is well-known and commonly practiced in the art. None of this work, however, describes combined micro- and sub-micrometer-scale field resolution, nor is there any provision for topographical measurements taught.
There is also prior work in using the SPM for local detection of high frequencies but with a mechanism distinct from that of the present invention. In this prior art, taught by Hou, Ho et al., "Picosecond electrical sampling using a scanning force microscope," Electronics Letters, 28 2302-3 (1992), the dynamic interaction between tip voltage and sample voltage is detected as a difference-frequency product of the two voltages in the displacement of the SPM cantilever, up to its fundamental mechanical resonance, typically 10-100 kHz. While this makes for a relatively fast detection mechanism, it suffers from three significant drawbacks. The first is that, as described in this reference, either topographical information or voltage information is acquired (but not both simultaneously, since they operate using the same signal); when voltage information is being acquired, the control loop which regulates the tip-sample distance must be opened, allowing for drift of this distance and possible loss of calibration. The second is that, by use of a standard SFM tip, the interaction of the voltage on the tip with structures or signals on the sample is significant-there is no electrical shielding of the tip or cantilever taught, unlike in the present invention. Finally, the technique of the prior art is limited to coherent detection, i.e. the voltage signal on the tip must have a common phase with that on the sample so that the (much lower) difference frequency can be detected by the motion of the SFM cantilever. This is a significant drawback which requires expensive instrumentation for generating these coherent measurement signals, and it limits the application largely to probing active electrical circuits, not passive samples.